Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) method and apparatus, in particular to a longitudinal relaxation time MRI method and apparatus.
Description of the Prior Art
MRI is an imaging technology involving biomagnetics and nuclear spin that has advanced rapidly with the development of computer technology, electronic circuit technology and superconductor technology. In MRI, human body tissue is positioned in a static magnetic field B0, then a radio frequency (RF) pulse with a frequency equal to the precession frequency of hydrogen atomic nuclei is used to excite the hydrogen nuclei by producing an excitation field B1 in the body tissue, giving rise to resonance in the hydrogen nuclei, with the absorption of energy. When the RF pulse has stopped, the hydrogen nuclei have been deflected (flipped) from an orientation along the filed lines of the B0 field, and as they return to that orientation they emit radio signals at a specific frequency, releasing the absorbed energy. These signals are received and recorded by a receiver outside the body, and processed by computer to obtain an image.
Non-uniformity of the excitation magnetic field B1 in a high-field MRI system often leads to inconsistency between the actual flip angle (FA) and the nominal (expected) flip angle, and this in turn has a serious impact on qualitative and quantitative analysis of the magnetic resonance image such that the diagnosis result is negatively affected. Because the linear relationship between the excitation magnetic field B1 and the RF pulse flip angle is generally determined by analysis or BLOCH simulation, a stable and accurate map of the spatial distribution of the excitation magnetic field B1 is important to rectifying the effect mentioned above (the inconsistency between the actual flip angle and nominal flip angle).